When controlling a wheel brake, in some applications, it is important to know when the brake pads (or linings) lift off from the brake disc or drum. This holds true, for example, in connection with the adjustment of the clearance (air gap). Another application can be seen in the control of the wheel brake when the brake releases, it being possible to ensure that the vehicle wheel brakes are released essentially simultaneously. In the following, the problem of adjusting the clearance is discussed as the preferred field of application.
The adjustment of the clearance at a wheel brake is desirable with a view to the continually existing wear and tear of the brake pad, as well as changes in the marginal conditions such as the brake temperature. This holds true in particular for motor-vehicle wheel-brake systems operated using electric motors, in which the wheel brakes are controlled by driving brake actuators having electric motors. Motor-vehicle wheel brakes operated by electric motors are known, for example, from PCT Publication No. 94/24453 or German Published Patent Application No. 19 526 645, where the application force is produced by an electric motor. In an electrically controlled braking system, a required clearance (distance between the brake pad and the brake disc) should be maintained in all working points in the unbraked state. For example, in the event of a lateral runout of the brake disc, a clearance which is too small leads to friction losses between the brake disc and the brake pad even in the unbraked state. If there is no compensation for the continually existing wear and tear of the brake pad, then a clearance which is too large will occur. This will result in a reduction of the dynamic response of the braking process, since after initiating the braking process, the electric motor would first of all have to run through the clearance before any braking action would come about at the brake disc. In addition, the goal should be for the clearance to be adjusted approximately equally at both wheels of a vehicle axle. A different clearance leads to a different response characteristic of the two wheel brakes of a vehicle axle during a braking process. In this context, one must expect a quicker braking action at the wheel having the smaller clearance than at the wheel having a larger clearance. Accordingly, the clearance has a not inconsiderable influence on the comfort and safety during the braking process of a motor vehicle.